1. Field of the Invention
The present invention relates to a process for producing a carboxylic acid ester, in which acid materials present in the esterified solution produced by an esterification reaction is neutralized.
2. Description of the Related Art
Conventional processes for producing esters comprise cooling a reaction solution produced after esterification reaction to an appropriate temperature, neutralizing the esterified reaction solution with a relatively diluted aqueous solution of a strongly or weakly alkaline neutralizing agent, separating an ester phase from the aqueous phase, refining the ester phase by washing with water, removing alcohol, and distillating to produce an ester product. On the other hand, the aqueous phase was discharged from the system after an oil component present in the aqueous phase was recovered.
Specifically in the neutralizing step of the process, the produced ester may undergo hydrolysis under the highly alkaline conditions resulting in a poor yield. In order to overcome this problem, it was required to use a diluted alkaline solution and complete the neutralizing reaction within a short period of time. For this reason, depending upon the presence of catalysts at the time of esterification reaction and the type of neutralization apparatus, generally the alkaline neutralizing agent was used as an aqueous solution in the concentration of 5 to 10% by weight, which solution was used in a ratio of 0.3 to 1 by volume relative to the esterified reaction solution, thereby enhancing contact efficiency. On the other hand, excess neutralizing agents were neutralized and discarded without being recirculated and reused.
The one-pass neutralizing process using such diluted alkaline solutions as described above required a great deal of the alkaline neutralizing agents as well as a great amount of water causing loss of the alkaline neutralizing agents and an increased load of waste water, though it had a great suppressing effect on the hydrolysis reaction.
In addition, it had a disadvantages of increased production of sludge which is one of the most difficult problems in the ester production. This sludge consists of floating insolubles having a nucleus of metal compounds produced by corrosion of an apparatus at the interfaces between the ester phase and the aqueous phase at the time of the neutralizing treatment. When a larger amount of the sludge was produced, separability between the oil phase and the aqueous phase was quite diminished after the neutralization so that the waste water entailed a greater amount of the oil component resulting in a loss of ester as well as an increased load of waste water. In order to avoid this difficulty, the prior art techniques required a step of removing the oil component from the water phase by filtration or extraction with solvents.
An attempt has been proposed to improve the neutralization step as disclosed in, for example, Japanese Patent KOKOKU (Post-Exam.) No. Sho 45-4974, where in the first stage, a proportion of 80 to 90% of the acid component present in the reaction solution after the esterification is neutralized with an aqueous solution of caustic soda or caustic potash having a concentration of 2 to 5% by weight with sufficiently mixing for a retention time of one minute or less, and then in the second stage, to the resultant mixture an aqueous solution of sodium carbonate or sodium bicarbonate having a concentration of 4 to 12% by weight is added with sufficiently mixing for a retention time of one minute or less to complete the neutralization of the acid component in the reaction solution. In this method, it is possible to some extent to suppress both the reduction in contact efficiency due to foaming with carbonic acid gas and the loss of the once produced ester due to the hydrolysis thereof. However, the neutralization step is complicated and in addition, such method is essentially the same in that a higher level of the alkaline neutralizing agent and a great amount of water are again required.
We have made an intensive research for novel process for neutralization capable of overcoming the problems of the prior art neutralization process using larger amounts of a diluted aqueous alkaline solution and water. As a result, we have found that the problems can be solved by using a countercurrently contacting column for neutralization, into which an aqueous strong alkaline solution and an aqueous weak alkaline solution are introduced at a middle portion and an upper portion, respectively, and contacted countercurrently with a reaction solution introduced at the bottom portion of the column, from which the present invention resulted.